Power semi-conductor switches which are able to switch electric currents of more than 1 A are characterized by having a simple control system and a high service life since they do not comprise parts tending toward wearing. Such power semi-conductors are in particular applied for switching supply voltages and as dimmers in order to continuously adjust the power of electric lamps, light fixtures or motors.
Apart from thyristors and bipolar transistors, field-effect transistors, which are characterized by a rapid switching behavior, are increasingly used as power semi-conductors. Furthermore, the positive temperature coefficient is advantageous in field-effect transistors, the positive temperature coefficient allowing for the implementation of a simple parallel circuit. Above all, n-channel field-effect transistors with a drift range in the area of the drain electrode are applied as power field-effect transistors.
Field-effect transistors may be operated as forward phase control dimmers and as well as reverse phase control dimmers for controlling the power emitted from an AC voltage source to a consumer. In forward phase controlling, the power emitted to the consumer by the AC voltage source is reduced by the fact that the beginning of the positive or the negative half-wave, respectively, is cut off according to the power to be controlled. In reverse phase controlling, in contrast, the power is reduced in the consumer in that the AC voltage is, beginning at the voltage pass through zero, switched through to the consumer delayed according to the power to be controlled or interrupted ahead of time, respectively. The reverse phase controlling is of the advantage, compared to the forward phase control, that no steep voltage edges occur when switched on.
Power circuits for switching and dimming AC voltages with field-effect transistors, as semi-conductor switches typically comprise two n-channel field-effect transistors connected in series with a drift range in the area of the drain electrode, the source electrodes of the two field-effected transistors being connected to each other. The drain electrode of the one field-effect transistor is then connected to the AC voltage source, and the drain electrode of the other field-effect transistor is connected to the consumer. The gate electrodes of the two field-effect transistors are connected to a control system controlling the gate voltage in order to bias into conduction and bias into cut-off the field-effect transistors and to thus adjust the power emitted from the AC voltage to the consumer.
In order to render the power circuit with the field-effect transistors arranged in series short-circuit proof, i.e. to prevent, on occurrence of a short-circuit in the consumer, that the power indicated by the AC voltage source damages or destroys the field-effect transistors by overheating, the power circuits normally comprise an additional short-circuit protection circuit.
According to the circuit configuration as commonly used, an additional ohmic resistor is connected in the current path for achieving the short circuit strength between the two source electrodes of the field-effect transistors. The control system connected to the gate electrodes of the field-effect transistors scans the voltage drop via this ohmic resistor. When the voltage drop increases considerably due to a short-circuit in the down-stream consumer, this voltage drop is registered by the control system and thereupon, the two field-effect transistors are switched off. This conventional short-circuit protection circuit leads to an undesired power loss due to the ohmic resistor additionally connected to the current path.
In order to prevent such a power loss, a short-circuit protection circuit for a forward phase control mode of a power circuit with two field-effect transistors being connected in series via the source electrodes is provided. In this short-circuit protection circuit, additional components for measuring the variation of the gate electrode potential in relation to the voltages applied between the source electrodes and the drain electrodes are provided. On the basis of the measuring results, the switching of the field-effect transistors into the conducting state is either allowed or prevented. This known short-circuit protection circuit is, however, complex and only applicable in the context of forward phase controlling.